#include<stdio.h>
#include<string.h>
int main()
{
	char arr[ ]="hello";
	strcat(arr, arr);//exat 
	/*
	1.strcat(destination,source)
	2.the source must be enough big
	3.the destination must be able to be changed
	4.the first operation will destory '\0',leading to endless operaion 
	5.it will change the source
	*/
	printf("%s ",arr);
	//hellohelloh... 
	return 0;
}